1. Field of the Invention
The present invention relates to an optical scanning device loaded in an electrophotographic image forming apparatus or the like.
2. Description of the Related Art
An electrophotographic image forming apparatus such as a digital copier, a laser printer or the like has an optical scanning device which deflects a light beam from a laser light source in a main scanning direction through a deflector, and forms a beam spot on a surface of a photosensitive body (surface to be scanned) moving in a sub-scanning direction (perpendicular to the main scanning direction). Thus, the optical scanning device forms a latent image of an image to be output on the surface of the photosensitive body.
As the deflector of the optical scanning device, a mechanical deflector employing a polygon mirror, a galvano-mirror or the like is used in general.
FIG. 1 shows an example of configuration of an optical scanning device in the related art.
The optical scanning device 40 causes a light beam 42 emitted from a laser light source 41 to be incident on a polygon mirror 47 rotating at high speed through a first optical system 46 consisting of a coupling lens 43, a diaphragm 44 and a line-image forming optical system 45, and causes the light beam to be reflected by a mirror surface 47a of the polygon mirror 47, the angle of which surface changes with the rotation of the polygon mirror 47. Thereby, the light beam 42 is deflected in the main scanning direction, and, thus, performs scanning repeatedly.
The light beam 48 reflected by the polygon mirror 47 passes through a second optical system consisting of an fxcex8 lens 49, a long-dimension lens 50 and a turning mirror 51, and is used to scan a surface 53a (surface to be scanned) of a photosensitive body belt 53 moving in the sub-scanning direction.
At this time, the laser light source 41 is intensity-modulated according to an image to be output, and, thereby, an electrostatic latent image is formed on the surface 53a of the photosensitive body belt 53 in a form of a dot pattern by the blinking light beam 48.
As mentioned above, a mechanical deflector employing a polygon mirror or a galvano-mirror is used as a deflector of an optical scanning device in general. Because each of the polygon mirror and galvano-mirror is driven by a motor, heat is generated by the motor when it is driving the mirror.
Recently, as an image output speed is increased, a driving speed of a deflector of an optical scanning device is increased. Thereby, a heat amount generated by the motor of the deflector increases. Accordingly, it becomes not possible to ignore influence thereof on other optical components (a light source, a coupling lens, a diaphragm, a line-image forming optical system, an fxcex8 lens, a long-dimension lens, and so forth) contained in a housing of the optical scanning device together with the deflector.
Heat generated by the motor is transmitted to the respective optical components mainly through the housing. Thereby, the respective optical components are heated, and, thereby, problems may occur such as a shift of imaged position (focus shift), thickening of light beam, error of writing magnification, and so forth. As a result, image quality may be degraded.
Further, as a driving speed of a deflector is increased, a vibration generated thereby when the deflector is driven has come to be not able to be ignored. Accordingly, it has come to be not possible to use a plastic as a housing of an optical scanning device, and a metal has come to be employed therefor. Because metal has a heat conductivity larger than that of plastic, heat from a motor of a deflector comes to be more easily transmitted by the housing. As a result, other respective optical components come to be affected by the heat from the motor of the deflector more seriously. Accordingly, the above-mentioned problems occur more remarkably.
Further, recently, lenses and/or mirrors which are main components of optical systems of an optical scanning device come to be made by plastic, and are made to have spherical surfaces so that both high performance and low costs of the device come to be directed.
However, because plastic-made components are not superior in heat-resistant property in comparison to glass-made components, lenses and mirrors which are main components of optical systems are more seriously affected by heat from a motor.
Accordingly, the above-mentioned problems occur more remarkably.
In the related art, Japanese Laid-Open Patent Application No. 7-244249 discloses an idea for avoiding influence of heat from a deflector.
According to Japanese Laid-Open Patent Application No. 7-244249, a scanning lens is separated from other optical components (mainly a deflector) in a housing, and, thereby, shift of scanning position and change of diameter of light beam occurring due to shake of air surrounding the scanning lens are prevented. This technique is directed to eliminate influence of shake of air (air flow generated by the deflector) around the deflector due to rotation of a mirror of the deflector, basically.
However, according to this idea, although the deflector is separated from optical components such as the scanning lens, a turning mirror and so forth which direct light reflected by the deflector to a surface to be scanned, the deflector is not separated from other optical components such as a coupling lens, a diaphragm, a line-image forming optical system which direct a light beam to the deflector. Accordingly, those other optical components are affected by heat generated by the deflector.
As a result, in the housing, some optical components are affected by heat from the deflector, and the other optical components are not. Generally speaking, in order to prevent characteristics of respective optical components included in a optical system from shifting unevenly, design is made such that the entirety of the optical system is in a uniform temperature environment, and, when the temperature environment changes, the system can be corrected as a whole. Accordingly, when some optical components are in a different temperature environment, a design for correction with respect to environment change becomes difficult.
The present invention has been devised in consideration of the above-mentioned problems, and, an object of the present invention is to provide an opitcal scanning device using a mechanical deflector employing a polygon mirror or a galvano-mirror in which heat generated by the mechanical deflector is made to be not easily transmitted to other optical components contained in a housing together with the mechanical deflector, and, thereby, stable optical scanning without fluctuation due top temperature can be performed.
An optical scanning device according to the present invention directs a light beam emitted from a light source to a mirror of a mechanical deflector through a first optical system, deflects the light beam in a main scanning direction by causing the light beam to be reflected by a mirror surface of the mirror, the angle of the mirror surface changing due to rotation of the mirror, and directs the deflected light beam through a second optical system to a surface to be scanned moving in a sub-scanning direction, the light source, first optical system, mechanical deflector and second optical system being contained in a housing. The mechanical deflector is held in the housing through a holding member, and, also, material of the housing is different in heat conductivity from the holding member.
Thereby, it is hard for heat generated by the mechanical deflector to be transmitted to the housing. Accordingly, it is possible to prevent the other optical components contained in the housing together with the mechanical deflector from being heated, and to prevent the performance of the optical scanning device from being degraded due to temperature rise of those optical components.
It is preferable that heat conductivity of the housing is smaller than heat conductivity of the holding member holding the mechanical deflector which is a heat source.
Thereby, it is head for heat generated by the mechanical deflector to be transmitted to the housing. Accordingly, it is possible to more effectively prevent the other optical components contained in the housing together with the mechanical deflector from being heated, and to prevent the performance of the optical scanning device from being degraded due to temperature rise of those optical components.
An optical scanning device according to another aspect of the present invention directs a light beam emitted from a light source to a mirror of a mechanical deflector through a first optical system, deflects the light beam in a main scanning direction by causing the light beam to be reflected by a mirror surface of the mirror, the angle of the mirror surface changing due to rotation of the mirror, and directs the deflected light beam through a second optical system to a surface to be scanned moving in a sub-scanning direction, the light source, first optical system, mechanical deflector and second optical system being contained in a housing. The mechanical deflector is directly mounted to the housing, and, also, material of the housing has heat conductivity smaller than that of a part of the mechanical deflector in contact with the housing.
Thereby, it is hard for heat generated by the mechanical deflector to be transmitted to the housing. Accordingly, it is possible to prevent the other optical components contained in the housing together with the mechanical deflector from being heated, and to prevent the performance of the optical scanning device from being degraded due to temperature rise of those optical components.
It is preferable that the mechanical deflector is covered by a cover having an optical window, and, thereby, an air flow occurring due to rotation of the mirror is kept within the cover.
Thereby, it is possible to prevent temperature rise of the other optical components due to convection of heated air, and, thereby, to prevent the performance of the optical scanning device from being degraded. Further, because an air flow occurring due to rotation of the mirror is prevented from reaching the other optical components, shift of scanning position, change of diameter of light beam and so forth occurring due to shake of air in the proximity of those optical components can be prevented.
It is preferable that a cooling part forcibly cooling the mechanical deflector is provided.
Thereby, it is possible to prevent the housing and air in the housing from being heated by heat from the mechanical deflector, and to prevent temperature rise of the other optical components.